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* The shearing-box (SB) approximation and its
viability, for numerically simulating MRI-induced turbulence in accretion
disks, is critically examined. It is shown that this approximation is
inadequate for the above purpose, due to its inability to deal with large
(inertial) as well as very small (dissipation) scales and the possible
introduction of spurious results (channel flows). The implications of
this result for accretion disk numerical simulations are discussed. With O.M. Umurhan (2008) - pdf
reprint (A&A) ;
* Saturation properties of the linear
magneto-rotational instability (MRI) are investigated, using asymptotic
techniques close to the instability threshold (weakly nonlinear limit).
The resulting envelope equation is found to be the real
Ginzburg-Landau equation. It is found that for small magnetic Prandtl
number (Pm) the saturation amplitude scales as Pm to a positive power and thus for a fixed
conductivity the angular momentum transport in the saturated state
decreases with increasing Re (the Reynolds number of the flow). The
implications of this result for accretion disk theory, MRI experiments
and numerical simulations are discussed.
With O.M. Umurhan & K. Menou (2007) - pdf
reprint 1 (PRL) ; pdf reprint 2 (PRE)
* Transient growth is found in global
axisymmetric dynamical disturbances of a thin polytropic accretion
disk. Asymptotic expansions are used in order to find analytical solutions,
giving the spatio-temporal structure of the transiently growing modes.
With O.M. Umurhan, A. Nemirovsky & G. Shaviv (2006) - pdf reprint (A&A)
* Nonlinear instability triggered by transiently growing linearly
stable modes gives rise to the formation of coherent vortices on accretion
disks. The vorticity distribution in a section of an accretion disk (a
"shearing box"), as a function of time, is shown in this movie (download and play in your
computer). An initially random distribution gradually develops into a
small number of persisting coherent vortices. Work done with O.M. Umurhan
(2004) - pdf
reprint (A&A)
* Are
ordered large-scale magnetic fields always necessary for the launching of
astrophysical jets? We examine the possibility that rapid rotational
energy liberation in the inner boundary layer of an accretion disk and
its conversion to internal energy (through strong shocks) may cause, if
cooling is not too efficient, ejection. The conditions for this mechanism
to be viable are assessed. Work done with N. Soker (2003) - pdf reprint (A&A)
* Backflows
in the meridional flow pattern of accretion disks have been shown to
exist, for sufficiently low values of the viscosity parameter (Urpin,
Kluzniak & Kita). We numerically show that the asymptotic analysis
results for polytropes are valid also for disks in which radiative
transport (in the diffusion approximation) is included. Work done with L.
Gitelman (2002) - pdf
reprint (A&A)
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